Those tags are terminated with [/...]
These are effective now in the Quick Reply box and in the rich text editor. Note that if you are using the rich text editor you should be using the toolbar icons instead. Also note that when you paste a URL in the rich editor it will automatically hyperlink it. In this case the tag will be incorrect - so you should use either the rich text auto hyperlink feature or the [url] tag.
Note that when you use the [img] tag in the rich text editor you might see the automatic hyperlink happening. In this case select the hyperlink and click the button to break it, otherwise the [img] tag will be corrupted.
This is to say that if you see the rich text editor you should really be using it - click the buttons in the toolbar to add elements.